SE464543B - CONTROL SYSTEM FOR A FRONT PANEL - Google Patents

Description

464 543 2 there is an oxidation zone, where carbon is converted to carbon dioxide (CO 2) and where carbon dioxide can be reduced to carbon monoxide (CO). There are additional zones. In any case, there is a very complex set of ongoing chemical reactions, which vary from boiler to boiler, depending on many parameters. Soda boilers have analogous reactions.

In order to achieve an optimal degree of combustion, flue gas analyzers have been developed, which measure the amount of carbon monoxide, carbon dioxide and also combustible substances (CHX).

In addition, of course, measurements have been carried out for the Environmental Protection Agency (EPA) on nitrogen (I) or nitrogen (II) oxides, sulfur dioxide and opacity (which is a measure of the soot or ash content of the flue gas). In addition, feedback techniques have either been proposed or actually used, according to which some of the above parameters have been used to regulate the efficiency of combustion. For example, the North American Combustion Handbook, 1978, points out the second edition, published by the North American Manufacturing Company, on p. 67 and 68, that an optimum in terms of thermal efficiency can be achieved by ensuring that the maximum content of carbon dioxide is present in the flue gas. Furthermore, regulation of the under-air flow according to the amount of carbon monoxide or oxygen in the exhaust gas has also been performed to set a selected target value.

It is a general object of the invention to provide an improved system and method for optimizing the combustion in a steam boiler.

According to the invention, in order to achieve this object, a control system and a control method are provided for a boiler which generates steam and has a four-bed of fuel, where air is supplied under or at the bed (sub-air) to effect the primary combustion of the fuel in the bed. Air is supplied above the bed (excess air) to complete the combustion. This system includes means associated with the boiler chimney for sensing carbon dioxide and carbon monoxide in the flue gas. The amount of 464 543 '3 sub-air to the boiler is regulated depending on the carbon dioxide or the steam / fuel ratio. The amount of excess air to the boiler is regulated depending on the amount of carbon monoxide.

The invention is described in more detail in the following with reference to the accompanying drawings. Fig. 1 is a schematic view of a stoker boiler according to the invention.

Fig. 2 is a detailed cross-sectional view of a stoker boiler of the type schematically shown in Fig. 1. Fig. 3 is a circuit schematically illustrating a control system according to the invention. Fig. 4 is a diagram illustrating the operation of the control system of Fig. 3. Fig. 5 is a table illustrating the operation of the control system of Fig. 3. Fig. 6 is a diagram of a portion of the air supply to a boiler and illustrates an alternative embodiment of the invention. Fig. 7 is a schematic view of a recovery boiler using the invention.

Reference is now made to Fig. 1. This shows a stoker-type power-generating boiler 10, where fuel, such as coal or bark, is fed at 11 on a movable grate 12. Combustion is maintained by means of upper air at 13 and lower air at 14. A blower fan ( FD) 16 provides such air.

The four-bed 17 on the grate 12 generates steam in steam pipes 18 and the amount of outgoing steam is shown at 19.

Flue gas is sucked out by means of a flue gas fan 21 and fed to a chimney 22. This has a flue gas analyzer 23, which comprises individual and known sensing units, indicating the amount of carbon monoxide (CO), carbon dioxide (CO 2), combustible substances (CHX) and the opacity (OP ) in the flue gas. These have the reference numbers 24-27. The control of the fuel supply is shown schematically with the gate unit 28, and the size of the feed is indicated at 29.

From the feed point of view, the amounts of upper and lower air are determined by means of sensors, the values being indicated at 31 and 32. The inputs for regulating these air flows via valves or dampers are indicated 464 3543 at 33 and 34.

The present invention can be applied to a spreading stoker according to Fig. 2. There is an air space at 41, with an under-air inlet 42, which space is covered by a movable stoker chain 43. The upper part of the chain supports the lighthouse 17, where excess air is supplied at the front. - and the rear end and the sides. The front air at the front is indicated at 44 and at the rear at 46 and 47. There is a carbon pocket 48 and a feeder 49, which throws fuel into the furnace. The upper part of the stoker chain 43 moves towards an ash pocket 51. now in such a boiler the fuel is generally thrown over the lighthouse with a uniform distribution. This enables suspension burning of fine fuel particles, and the heavier pieces, which cannot be supported by the gas flow, fall on the moving grate and burn in a thin, fast-burning bed, which moves towards the front of the boiler. This firing method gives extreme sensitivity to load fluctuations, as the ignition is almost instantaneous and the firing speed increases. In addition, the fuel bed can be burned out quickly, if desired.

Fig. 3 illustrates the control system for the boiler in Fig. 1.

At the right edge of Fig. 3, the different inputs and outputs are correlated. In other words, several sensors detect the steam, the fuel, the carbon dioxide, the opacity, the carbon monoxide and the combustible substances. The sensed values are processed in the manner described below, and by means of the measurement of the present sub-air (U.G.) and the upper air (O.F.) 31, 32, two control circuits are established for adjusting the respective air flows over the connections 33, 34.

The under-air control circuit is now treated, the purpose of which is to maximize detected carbon dioxide. At 25 detected amount of CO 2 is led to an extreme regulator 52, which, by even pitch function or pitch function, senses the maximum amount of carbon dioxide and changes the U.G. air at 33 accordingly. In other words, the variation of the carbon dioxide value as a function of the U.G. air is a curve, which has a maximum; and the U.G.-1uft 464 543 'value is varied until the maximum amount of carbon dioxide is measured. Such extreme control is shown in Fig. 5, where changes are made to the U.G. air (related to an assumed constant fuel supply). Regardless of whether the value of the most recent carbon dioxide measurement increases or decreases, a note is made until the extreme or maximum point is reached. Extreme regulation is in itself previously known, for example through an article entitled EXTREMUM CONTROL SYSTEMS-AN AREA FOR ADAPTIVE CONTROL? by Jan Sternsby, conducted in conjunction with the 1980 Joint Automatic Control Conference, August 13-15, 1980 in San Francisco, California.

The specific control techniques used here are similar to the "frying methods" described in this article.

This article also discusses other methods that can be used, such as a gradient technique (cf. Mode Oriented Methods).

As an alternative to the under-air control by measuring carbon dioxide, you can use the steam / fuel ratio indicated at 53. This is especially useful for boilers that have accurate measurements of fuel and steam flows, as well as for detecting certain undesirable conditions, such as fuel accumulation in the boiler. Thus, in general, CO 2 or the vapor / fuel ratio are determined either separately or in combination using their respective confidence levels. Of course, the steam / fuel ratio is a crucial measure of the boiler's efficiency, as it corresponds to the ratio between outside energy and inenergy.

Thus, in fact, a cross-limitation scheme with respect to the steam / fuel ratio is used to obtain variation in accordance with this ratio, where perhaps heterogeneous fuel bed ratios may justify this. The diagram according to Fig. 5 also shows this ratio or ratio (S / F) as an alternative to carbon dioxide.

An alternative method of extreme control involves finding a polynomial of the second degree for CO2 as a function of the preceding values of the air / fuel ratio and the fuel flow. A second quadratic polynomial for the steam / fuel ratio as a function of the preceding values of the air / fuel ratio and the fuel flow also exists. A recursive exponentially weighted least squares method, reproduced in Chapter 7-3-1 of the book DYNAMIC SYSTEM IDENTIFICATION by G.C. GO0dwin and R.L. Payne, Academic Press, pp. 180, 1977, was used to calculate (identify) the polynomial parameter coefficients.

This calculation theory is then used to find the expression for estimating the air / fuel ratios, where the maximum CO2 value and the maximum steam / fuel ratio are present.

For example, let the steam / fuel ratio polynomial be given by s / F = Al A / Fz + A2 A / F + A F + A4 A / F - F + A5 (1) 3 For Al <0, the maximum steam / fuel ratio is when d S / F _ _ T fi-0-2 AlA / F4-A2-1-A4F (2) ie, -A2 A4 FA / F at max S / F = EK- - ïÃ- (3) ll where S / F = steam / fuel ratio A / F = air-to-fuel ratio Ai = identified parameters for i = 1,2,3,4,5 The expression for the air / fuel ratio corresponding to maximum C02, A / F at maximum C02, can be written in a similar way as Equation 3 .

The extreme controller is then used for ramp increase / decrease in air / fuel ratio target in one of the following three ways: a) A / F at max S / F b) A / F at max C02 c) An algebraic combination of a) and b) 464 543 '7 The key feature of this controller is that these air / fuel ratio values change with variations in the composition operating conditions. The identification uses the current measurements to update the two square polynomial parameters, when the new measurements become known, and predicts the air-to-fuel ratio values for the I optimal steam / fuel ratio and CO2 evenly and constantly.

The extreme regulator 52 also has an opacity input 27, which is used to provide additional U.G. air if the 0-F inlet air is at a maximum. This is to comply with, for example, the regulations from the EPA (Environmental Protection Agency). The U.G. air indicator 31 is divided by the steam outflow 19 or the fuel inlet 29 and is added at 54 to the setting output signal of the regulator 52. This gives an error signal to controller C5 with respect to U.G. air / steam or U.G. air / fuel. This thus forms an intermediate control circuit. Finally, the innermost control circuit is formed by addition at 56, which receives the UG air input signal 31 and the output signal of the controller C5, which after processing in the control unit 57 is in fact a UG air error signal, which constitutes the UG air control line 33. .

Reference is still made to Fig. 3. Excess air (0.F.) is regulated by means of three parallel regulators C1, C2 and C3, of which only one is active at a time and which have respective inputs for setting values (SP) for combustible gases, carbon monoxide and opacity.

These are summed at 61, 62 and 63 with the actual values of these parameters. The choice of any of these three parameters to serve as a target for OF-air is indicated by a switch T. This choice is made by a set of logical equations for state transition, as illustrated in Table 1. The result on the line 64 is added at 66 to an input signal at 67, which is the ratio between either OF air and steam or 0.F. air and fuel. The resulting addition at I 66 is an over-air error signal, which is processed by the controller 464 543 8 C4. This thus forms an intermediate control circuit.

The final innermost control circuit for the OFF air signal 32 and the control output signal 34 is achieved by the addition unit 68, which receives the 0.F.-air signal 32, the output signal of the regulator C4 and gives rise to an OF-air error signal which goes to a regulator 69, which drives the OF air control line 34.

In general, the intermediate control circuit, which uses the quotient 67 for 0.F.-air / steam or O.F.-air / fuel, is not absolutely necessary for the control described.

Thus, and to partially summarize the present invention with reference to Fig. 3, the regulation of the sub-air, which can form as much as 80% of all combustion air in many boilers, is accomplished by measuring carbon dioxide (and / or the ratio of steam to fuel) alone . Of course, a measurement of the oxygen concentration would be equivalent. It is our belief that there is no theoretical justification in using carbon monoxide for this purpose. On the other hand, carbon monoxide (as will be discussed later as an alternative to combustible substances or opacity) is used to regulate the excess air. This is due to the fact that the presence of carbon monoxide in the flue gas is indicative of inappropriate mixing of excess air with carbon monoxide or of an approach to stoichiometric combustion conditions in the oxidation zone above the bed. It provides very limited information about the condition of the lighthouse itself. On the other hand, it is our belief that the carbon dioxide measurement (or alternatively steam / fuel) reveals more about the condition of the lighthouse or fuel bed. Thus, the above represents a partial summation of the reason for the control system design in Fig. 3. 4 464 543 TABLE IT = 0 or 1 or 2 Initialization: T = 1 ie CO control State transition logic: f T - + 0 if cnx> cn ooh op <opx ooh co <cox xx activate á CHx control 3 if CHX> CHXSP + CHDZ and OP <OPSP and CO <CO L SP f T -> 1 if co> cox ooh oP <oPx activate 24 COf control nl fl g if CO> COSP + CODZ and CHX <CHXSP and \ oP <opsp T -å 2 If OP> OPX activate 1 ïåâääëâëfïnn oP> oP + oP and ca <cH _ sp DZ x xsp T x PRIORITY Reference is now made to Table I and Fig. 4, which illustrates the logic equations for transition for selecting one of the three parallel control inputs for the excess air, cf. Fig. 3, ie combustible substances, carbon monoxide and opacity. The terms in the equations in Table I are equivalent to the designations in Fig. 4. The priority of the control is, as shown, in sequence opacity, carbon monoxide and CHX. In general, the carbon monoxide control is suppressed by the opacity control, if the opacity exceeds a predetermined limit value. The CHX control is in turn suppressed by the carbon monoxide control 464 545, if the sensed value of carbon monoxide exceeds a predetermined limit value.

All this is illustrated in Fig. 4, where, for example, when the carbon monoxide portion of the diagram is considered, the carbon monoxide setpoint (CO S.P.) includes a carbon monoxide dead zone (CODZ). Such a dead zone prevents throws.

Dead zones are also present in the other control channels. Maximum carbon monoxide level is indicated as COX, where an alarm condition prevails. The same applies to the maximum level of combustible substances, indicated as CHXX. In terms of opacity, the EPA limit level is indicated as OPX. Typical values given for the setting points are 0.1 - 1% for CHX, 200 - 1500 ppm for carbon monoxide and 10 - 20% for opacity. These values are of course dependent on the type of boiler used and the type of specific fuel. the values may also vary according to the present environmental protection regulations. For example, in the case of a certain boiler or in a certain type of bark fuel, the setting point of the carbon monoxide should be more critically adjusted to a relatively lower value than the other setting points. In each case, as is clear from this logical transition scheme, only one regulator is active at a time, in terms of excess air. Table II shows actual operating data for the present invention for three stoker boilers, one fired with bark and the other two with coal. 464 543 'll TABLE II FUEL TYPE gègg COAL # 1 KQL # 2 Oxygen Z 4.2 10.1 9.5 CO PPM 580 171 233 C02 Z 14.8 12.1 l2, l Opacity Z? * 31.2 - Combustible * Substances Z 0.1 0.1 0.1 Fuel flow MPPH 92 48 53 Steam flow MPPH 254 158 154 Sub-air flow MPPH 319 127 155 Steam / Fuel gain 2.76 3.29 2.91 Air / Fuel Z (Air / Steam) 139 , 5 (79) (100) Excess air Pressure Flow Flow 2.21 "water 55 MPPH 29 MPPH * Uses wet scrubbers, opacity not important from EPA point of view.

** Measurements unknown; stated values estimated.

In a spreader, the ignition plane moves upwards through the bed in the same direction as the sub- or primary air, which supplies the oxygen required for combustion. Volatiles are released directly into the supernatant zone for oxidation. Depending on the suspension burning of fine fuel particles and volatile substances, such boilers require a correct distribution of the secondary air (the upper air) in all load conditions. Improper air distribution results in a decrease in boiler efficiency through the formation of soot (with attendant opacity problems) and excessive transfer of fly ash and combustible hydrocarbons to the chimney. A slight fire around the bed also creates an increase in the proportion of carbon in the ash, via a loss of radiant energy towards the fuel bed from above. 464 543 i 12 In a spreader stoker, the firing plane is not well defined. Rather, it can be said to lie in two planes: l) at the root of the flame over the bed, where suspension combustion takes place, and 2) roughly parallel to the surface of the fuel bed. Volatiles are released directly to the secondary oxidation zone above the bed, when newly dropped carbon sinks to the ignition plane.

Since volatiles are allowed to reach the secondary oxidation zone in the diffuser stoker without having to undergo an ignition plane, a complete oxidation of these volatiles and the carbon monoxide rising from the fuel bed requires proper supply and distribution of the excess air.

Fig. 6 illustrates a diagram for controlling the distribution of the excess air. Here the main flow of excess air is indicated by the sensor 32 ', and this is controlled by a valve or a damper 83. Such a valve is normally controlled by the control signal 34 shown in Fig. 3. However, the secondary air inflow is divided by side, rear and front ducts. . At least the front and rear ducts are shown in Fig. 2 with the reference numerals 44, 47 and 47, respectively. In the side and rear ducts according to Fig. 6 there are controllable valves 81 and 82.

By using this duct system for excess air and valves or dampers to determine the distribution of air between the front, rear and side parts of the boiler, such a redistribution can significantly improve the efficiency of the boiler. As an aid to this distribution, the channel 26 for combustible substances can be used. This is connected to a controller 84, which performs a two-dimensional search over an acceptable range of excess air flows in order to minimize the CHX value. Thus, the controller 82 controls the control circuits 86 and 87, which control the dampers 81 and 82, respectively.

Feedback indications of the condition of these dampers are provided by units 88 and 89. Thus, by using the technique shown in Fig. 6, combustible substances can be minimized by controlling the distribution of secondary air. In addition, CO and opacity can be minimized in a similar way by controlling the distribution of excess air. Fig. 7 illustrates a recovery boiler which makes use of the principles according to the invention. A recovery boiler is generally used for the combustion of black liquor originating from a paper process. Spray nozzles 71 and 72, located on two sides of the furnace 73, emit black liquor in an atomized form in the furnace. The combustion air is supplied by means of blowers 74 and 74a and the air is divided, as illustrated, into a primary airway 75 and a secondary airway 76 and into certain types of recovery boilers into a tertiary airway 77. Appropriate air control valves 75a, 76a and 77a are used to control air volumes.

Primary air 75 is supplied via the valves 78 to the four-bed level. However, it can be treated in the same way as and can be called sub-air in this context of the invention. Similarly, secondary air 76 is supplied via the valves 79 and can be handled as excess air. Tertiary air 77 does not occur in all recovery boilers and can be handled as part of the secondary air for the purposes of this invention. From a control point of view, cf. Fig. 3, the primary air 75 and the secondary air 76, 77 are thus regulated in the same way as sub-air and upper air.

In summary, the present invention provides an improved boiler control system.

Claims (9)

464 543 l0 15 20 25 30 14 PATENT REQUIREMENTS 1. Control system for a steam boiler with a four-bed of fuel, where air is supplied under or near the four-bed (sub-air) for carrying out the primary combustion of the fuel in the four-bed and air is supplied over the four-bed (upper air) to complete the combustion, - designed by means (24, 25) for sensing carbon dioxide and carbon monoxide in the flue gas from the boiler chimney; a device (33) for controlling the amount of sub-air to the boiler as a function of the carbon dioxide measurement or the steam / fuel ratio; and a device (34) for controlling the amount of excess air to the boiler as a function of the carbon monoxide measurement. 2. A system according to claim 1, characterized in that the device for regulating the excess air is also controlled as a function of combustible substances in the flue gas or the opacity in the flue gas. 3. A system according to claim 2, characterized in that the opacity has the highest priority, the carbon monoxide has a second degree priority and the combustible substances have a third degree priority. A system according to claim 2, characterized in that the secondary air is supplied to the boiler at a plurality of places (76a, 77a) and that said control devices (33, 34) distribute the secondary air so that the combustible substances are minimized. System according to claim 1, characterized in that control changes are carried out only after the system has been stabilized from a previous change. System according to claim 1, characterized in that the boiler is of the recovery boiler type with at least primary and secondary air inlets (78, 79, 77) and that the lower air constitutes said primary air and the upper air constitutes said secondary air. I) 10 15 20 464 543 15 7. A method of regulating a steam boiler with a four-bed _ of fuel, where air is supplied under or near the four-bed (sub-air) for carrying out primary combustion of fuel in the four-bed and air is supplied over the four-bed (upper air) to complete the combustion, characterized by the following steps: that carbon dioxide and carbon dioxide are sensed in the flue gas; that the amount of under-air supplied to the boiler is regulated as a function of the carbon dioxide or steam / fuel ratio and that the amount of excess air to the boiler is regulated as a function of the amount of carbon monoxide sensed. 8. A method according to claim 7, characterized in that the amount of CO 2 is maximized and the amount of CO is adjusted to a target value. 9. Claim 7, characterized in that the opacity and the amount of CHX (combustible substances) are sensed and that the amount of excess air is also regulated as a function of said opacity and the amount of CHX as well as carbon monoxide (CO) and that the CO control is suppressed by the opacity control, if the sensed opacity value exceeds a predetermined limit value, and the CHX control is suppressed by the CO control, if the CO exceeds a predetermined limit value.